1. Field of the Invention
This invention relates to a limited-slip differential gear comprising a differential gear case which is adapted to be driven and in which two coaxial side gears and at least one pair of differential pinions cooperating with each other and associated with the side gears are rotatably mounted, wherein each differential pinion and the associated side gear constitute a worm gear train.
2. Description of the Prior Art
Such a limited-slip differential gear has a low efficiency because the friction has intentionally been increased so that a restraining action which depends on the driving torque will be effected unless the parts for performing the differential function and those for performing the locking function are structurally separated. Such a Torsen differential gear is a limited-slip differential gear in which the desired internal friction is produced by the side gears and differential pinions. Such Torsen differential gears have proved highly satisfactory because they are robust and can take up high loads. But as is apparent from EP-A-139 679 (WO 84/03747) the worms were used as side gears and the worm gears were used as differential pinions in said differential gears and the worm gears were constituted by pairs of parallel pinions and were operatively coupled by meshing spur gears. This means that a large number of gears were provided, which added to the manufacturing costs and to the bulk of the limited-slip differential.
EP-A2 148 641 discloses limited-slip differential gears which comprise cooperating parallel worms for performing a differential action between the side gears, which have helical teeth. Each pair of diametrically opposite ones of said worms cooperate with each other via a worm gear. That design also involves a high structural expenditure and the provision of worm gears disposed between the side gears and the fact that the differential pinions and side gears have parallel axes result in a particularly large space requirement. Similar remarks are applicable to the limited-slip differential gear which is disclosed in EP-B1 130 806 and comprises helical side gears and differential pinions meshing with each other. The differential pinions consisted of planet pinions, which were regularly distributed around the periphery and in mesh with the sun gears, which constituted the side gears. Said pinions were in mesh with each other in alternation. Owing to the provision of a large number of gears and pinions and the facts that the differential pinions and side gears had parallel axes and the sun gears were separated from each other by an internally arranged device for applying pressure, that differential gear was expensive and bulky.